1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling cell addition and release for operation of dual connectivity and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (CoMP) transmission/reception, UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Small cells controlled by low-power base stations (e.g., low power NBs/eNBs) are considered to solve fast-growing mobile traffic. The small cells can be deployed in hot spots for both indoor and outdoor scenarios. A low-power base station generally means a base station with a transmission power lower than that of a macro cell base station (e.g., normal NB/eNB). For example, a pico cell base station and a femto cell base station are usually considered low-power base stations. A UE may simultaneously communicate with a macro cell base station and a low-power base station, i.e. dual connectivity. In this situation, the UE can performs transmissions and/or receptions (e.g., of data, packets, messages and/or control information) via both the macro cell base station and the low-power base station. The dual connectivity provides benefits that the data throughput increases due to simultaneous data transmissions from/to the macro base station. However, it is unknown how to configure a cell of the low-power base station to the UE to enable the dual connectivity, i.e. adding a cell of the low-power base station while the UE has connected to the macro cell base station, which may have security concerns. In addition, it is also unknown how to release a cell of the low-power base station configured to the UE for the dual connectivity for the UE.
Thus, how to add a cell to enable the dual connectivity and release a cell in the dual connectivity and satisfy the security requirement are important problems to be solved.